The increasing quantity of sewage sludge from municipal and certain industrial sewage purification plants and the increasing technical and economic difficulties which are faced in eliminating this sludge, on the one hand, and the undisputed need for humus-forming substances in agriculture and areas of special cultivation, on the other hand, make it desirable to recycle the sludge. For this purpose the sewage sludge must be processed to form a usable humus fertilizer which is pure in hygienic terms, chemically harmless and easy to transport and spread. This applies also to all other waste materials suitable for decomposition.
This conversion of sewage sludge into humus fertilizer is best effected by aerobic stabilization and the rotting method which is generally termed composting.
Since practical experience has shown that static and semi-static composting according to the heap-method of composting and tower or silo composting--particularly for sludge alone and basically for all waste materials whose water content is appreciably more than that of the compost--are unsatisfactory both in terms of method and from the technical/economic aspect, new methods are tending to develop in the direction of machine composting, which ensures a very rapid decomposition process. In these methods the conditions required for composting, such as e.g. thorough mixing, ventilation or supply of oxygen, moisture content, temperature regulation, etc. can be better controlled so that composting can be carried out more reliably, more rapidly and considerably more advantageously from the technical/economic aspect.
In previously known sewage sludge composting processes the sludge, whether it be fresh or rotted down, is comminuted and homogenized if necessary before composting, fed into the composting plant and if necessary comminuted again during the composting process or before storage. Particularly when rotting towers of the non-tiered type are used for practising the known methods, there is a great danger of anaerobic zones forming as a result of insufficient mixing and partly for this reason quite considerable quantities of loosening agents have to be added to ensure a relatively slow rotting process.
Moreover, there is also a known method for rapid machine composting comprising a rotary drum for receiving the material to be composted wherein the latter is mixed with the sludge compost, already in optimum rotting condition in the rotary drum, in such a manner that always a smaller quantity of material to be composted is absorbed by a greater quantity of sludge compost, and the resultant mass or compound is kept in motion while maintaining optimum conditions for composting. Very good results have already been obtained with this method. However, it is still of disadvantage in this method that the material to be composted, which is mixed with the sludge compost already in the drum, frequently does not have uniform and optimum quality.